Nitroxyl spin probe studies often result in superimposed electron paramagnetic resonance (EPR) spectra respresenting the different environments probed by the nitroxyl radical. It has been observed that addition of a paramagnetic metal to the system can broaden the EPR signal of the nitroxyl radical in one of the environments, thus facilitating observation of the nitroxyl in the other environment. Among the paramagnetic metal species which have been found useful for this purpose the advantages of Fe(CN)63- have been emphasized. Using theoretical results as a guide we have found that several chromium (III) complexes are more effective than Fe(CN)63-. It is proposed to determine the relative contributions of exchange and dipolar interactions to broadening under a wide variety of conditions likely to be encountered in biological systems. Additional reagents will be developed to optimize the broadening of nitroxyl spin probes for a diversity of applications now evident.